BUG: Fix passing non-existant file to read_msgpack pandas-dev#15296

Author: chrisburr

doc/source/whatsnew/v0.21.0.txt

@@ -346,6 +346,7 @@ I/O
 - Bug in :func:`read_csv` when called with a single-element list ``header`` would return a ``DataFrame`` of all NaN values (:issue:`7757`)
 - Bug in :func:`read_stata` where value labels could not be read when using an iterator (:issue:`16923`)
 - Bug in :func:`read_html` where import check fails when run in multiple threads (:issue:`16928`)
+- Bug in ``pd.read_msgpack()`` with a non existent file is passed in Python 2 (:issue:`15296`)
 
 Plotting
 ^^^^^^^^

pandas/io/packers.py

@@ -193,7 +193,6 @@ def read(fh):
 
     # see if we have an actual file
     if isinstance(path_or_buf, compat.string_types):
-
         try:
             exists = os.path.exists(path_or_buf)
         except (TypeError, ValueError):
@@ -203,18 +202,21 @@ def read(fh):
             with open(path_or_buf, 'rb') as fh:
                 return read(fh)
 
-    # treat as a binary-like
     if isinstance(path_or_buf, compat.binary_type):
+        # treat as a binary-like
         fh = None
         try:
-            fh = compat.BytesIO(path_or_buf)
-            return read(fh)
+            # We can't distinguish between a path and a buffer of bytes in
+            # Python 2 so instead assume the first byte of a valid path is
+            # less than 0x80.
+            if compat.PY3 or ord(path_or_buf[0]) >= 0x80:
+                fh = compat.BytesIO(path_or_buf)
+                return read(fh)
         finally:
             if fh is not None:
                 fh.close()
-
-    # a buffer like
-    if hasattr(path_or_buf, 'read') and compat.callable(path_or_buf.read):
+    elif hasattr(path_or_buf, 'read') and compat.callable(path_or_buf.read):
+        # treat as a buffer like
         return read(path_or_buf)
 
     raise ValueError('path_or_buf needs to be a string file path or file-like')